Clean rooms are utilized in many industries for contamination control and to improve product quality and product yields. By way of example, clean rooms may be employed in pharmaceutical, biotechnology and semiconductor applications. Semiconductor manufacturing environments will be used hereinbelow and shall serve as an illustrative environment.
Airborne contaminants must be reduced, eliminated or both to help ensure optimum semiconductor yields. Therefore, gas filtration is critical in semiconductor manufacturing environments. Tremendous efforts are made to eliminate yield-reducing contaminants from the gases used in semiconductor processing tools. Contaminants can generally be classified as either particulate or molecular. Common particulate contaminants include dust, lint, dead skin and manufacturing debris. Examples of yield reducing contaminants include acids, such as, for example, hydrobromic acid, nitric acid, sulfuric acid, phosphoric acid, hydrochloric acid, bases, such as, for example, ammonia, ammonium hydroxide, tetramehtlyammonium hydroxide, trimethylamine, triethylamine, hexamethyldisilazane, N-methylpyrrolidone (NMP), cyclohexylamine, diehtylaminoethanol, methylaamine, dimethylamine, ethanolamine, morpholine, condensables, such as, for example, silicones and hydrocarbons with a boiling point greater than or equal to about 150° C. and dopants, such as, for example, boron, usually as boric acid, phosphorous, usually as organophosphate, and arsenic, usually as an arsenate.
Airborne particulate contaminants may be present in the ambient atmosphere within a clean room or they may be introduced by way of gases injected therein. For example, in semiconductor photolithography tools, gas is supplied for generally two purposes, namely, the actuation of tool pneumatics and the purging of tool optics. Although purified dry air, nitrogen or the like is generally used to drive pneumatics and purge optics, small amounts of contaminants are still liable to be present in the gas at concentrations sufficient to damage tool optics, for example, illuminator optics and projection lenses. Contaminating substances may adhere onto the optical elements to form molecular films. Molecular films on optical surfaces physically absorb and scatter incoming light. Scattered or absorbed light in photolithography optical surfaces causes distortion of the spherical quality of wavefronts. When the information contained in the spherical wavefront is distorted, the resulting image is also misformed or abberated. Image distortions, or in the case of photolithography, the inability to accurately reproduce the circuit pattern on the reticle, cause a loss of critical dimension control and process yield.
Contaminating substances may also chemically react with the optical surfaces of the photolithography tool, the wafer being processed in the tool or both. For example, sulfur dioxide may combine with water in the tool to produce sulfuric acid, which can irreversibly damage tool optics. In addition, ammonia may react with wafer surface materials such as the resist, gate-insulating films and the like. Such a reaction can interfere with the photolithography processing step and reduce process yields. Thus, the purity of the gases supplied to semiconductor processing tools is of critical concern.
The quality of gas flow and such yield-reducing contaminants in a clean room are often monitored through several sampling ports, for example, for one or more semiconductor processing tools. Typically, these sampling ports are used so as to ensure that a clean fabrication environment is being maintained and that semiconductor yields are not affected by an increased contaminant(s) level. Deviations in gas flow quality can also significantly affect semiconductor yields. Contaminants and variable gas flows may also increase maintenance and operational costs. Typical sampling ports for monitoring gas flow quality tend to plug during, for example, wafer fabrication, which can hinder process control and quality assurance efforts. Opening a sampling port during fabrication can also cause detrimental changes in pressure.
Conventional ports also require a substantial time period for sampling to occur as ports are commonly purged for several minutes to expel any residual contaminants that may affect quality measurements. Additionally, the gas flow rate through such ports often varies during sampling, which poses problems with sample uniformity and accuracy. In order to improve semiconductor yields and monitor yield-reducing contaminants, it is necessary to be able to conveniently sample gas flows without any of the aforementioned shortcomings of conventional ports. It is also necessary to be able to reduce and/or eliminate such contaminants. A convenient means for gas flow sampling and reducing or eliminating yield-reducing contaminants should also be easily adaptable to already existing semiconductor tools, clean rooms and their associated sampling ports. It may also be useful to continuously remove yield-reducing contaminants and monitor gas flow quality for such contaminants.